A pyrolytic boron nitride heating unit includes a dielectric body of boron nitride and a heating element contained within the boron nitride body. The heating element is composed of a conductive material preferably of graphite and more particularly pyrolytic graphite. A pyrolytic boron nitride heating unit may be fabricated by depositing a layer of a conductive heating material over a substrate of pyrolytic boron nitride over which is deposited an outer layer of pyrolytic boron nitride as taught in U.S. Pat. No. 5,343,022 the disclosure of which is herein incorporated by reference. The outer layer of pyrolytic boron nitride electrically insulates the surface of the heating element and provides protection to the heating element from mechanical damage.
The heating element is connected to a power supply causing electricity to pass through the conductive material which, in turn, generates thermal energy by resistive heating. In most applications heat transfer is accomplished by conduction when the member or device to be heated is placed in physical contact with the heating element. It is, however, not always practical or even possible to establish physical contact between the heating element and the device or member to be heated. In these instances heat transfer must be accomplished by either convection or radiation. One example in which neither conduction or convection may be used for heat transfer is in molecular beam epitaxy "MBE" which is a process useful for growing semiconductor films. In this process the semiconductor film is grown in a relatively high vacuum environment where heat transfer by convection is not possible and heat transfer by conduction is not practical. Accordingly, in this application, which is only one of many, radiation is the only available heat transfer mechanism.
The efficiency of the heating unit to transmit thermal energy by radiation is directly proportional to the emissivity of the heating element in the heating unit. The emissivity of the heating element is defined as the ratio of the emissive power of an actual surface of the heating element to the emissive power of an ideal black body at the same temperature. The emissivity of commercially available pyrolytic boron nitride heating units has been measured at 0.55 at a wavelength of 1.55 microns. For purposes of comparison an ideal black body at the same wavelength would have a radiation thermal efficiency of 100% representing a measurement of 1.00. Accordingly, commercially available pyrolytic boron nitride heating units are not very efficient for use as radiant heaters.